The present invention relates to an optical element and a method for manufacturing the same, and particularly relates to an optical element that enables recording and reproduction of a stereoscopic image as a hologram.
Holography arts have been known from a long time ago as methods for recording and reproducing a stereoscopic image onto and from a medium, and holograms prepared by such methods are used in various fields, such as ornamental art, forgery prevention seals, etc. In a general method for preparing a hologram optically, interference fringes of an object light, which is emitted from an object, and a reference light are recorded onto a photosensitive medium. Normally, laser light, which is excellent in coherence, is used as a light source for the object light and the reference light. In general, the behavior of light and other electromagnetic waves can be grasped in the form of propagation of a wavefront, having an amplitude and a phase, and a hologram can be said to be an optical element having a function of reproducing such wavefronts. Thus in a hologram recording medium, information for accurately reproducing phases and amplitudes of the object light at respective positions in space must be recorded. By recording interference fringes, resulting from an object light and a reference light, on a photosensitive medium, information that include both phases and amplitudes of the object light can be recorded, and by illuminating this medium with a reconstruction illumination light equivalent to the reference light, a portion of the reconstruction illumination light can be observed as light with wavefronts equivalent to those of the object light.
In preparing a hologram by such an optical method using laser light, etc., the phases and amplitudes of the object light can be recorded only in the form of interference fringes with the reference light. This is because the photosensitive medium on which the hologram is recorded has the characteristic of becoming photosensitized according to light intensity. Meanwhile, techniques for preparing holograms by computation using a computer have come to be put to practical use recently. Such techniques are referred to as CGHs (computer generated holograms), with which a computer is used to calculate wavefronts of an object light and a hologram is prepared by recording the phases and amplitudes of the object light by some means on a physical medium. By using such a computer generated hologram technique, not only can an image obviously be recorded in the form of interference fringes of an object light and a reference light but information concerning the phases and amplitudes of the object light can also be recorded directly onto a recording plane without the use of the reference light.
For example, U.S. Pat. Nos. 6,618,190, 6,934,074 and WO 2005/073816A1 (patent applications of inventions by the same inventor as that of the present application) disclose an invention, with which an arbitrary original image and a recording plane, having representative points positioned at a predetermined pitch, are defined, and a computer is used to calculate, for each representative point position, a complex amplitude of a wavefront of a synthetic wave of object light components emitted from respective portions of the original image, determine a complex amplitude distribution (distribution of amplitudes A and phases θ) on the recording plane, and record the complex amplitude distribution by means of a set of three-dimensional cells. The invention disclosed in the above-mentioned publications shall be referred to hereinafter as the “prior invention.” With the method disclosed in the prior invention, three-dimensional cells, each having a groove on one surface, are prepared, and by positioning these cells at each representative point position, an optical element, constituted of a set of a large number of three-dimensional cells, is arranged. In this process, the phase θ, determined for an individual representative point position, is recorded as a depth of the groove of the corresponding three-dimensional cell, and the amplitude A is recorded as the width of the groove of the three-dimensional cell. Because a unique phase θ and amplitude A are thus recorded at each individual representative point position on the recording plane, when a reconstruction illumination light is illuminated, a hologram reproduction image of the original image is obtained.
Because the optical element is constituted of the set of the three-dimensional cells, in which the amplitudes A and the phases θ are recorded, the method of the prior invention described above provides the merit of enabling manufacture of an element that can provide a high diffraction efficiency during reproduction. However, microfabrication techniques and a manufacturing process of high precision are required for physically manufacturing an optical element constituted of such three-dimensional cells. For example, the abovementioned U.S. Pat. No. 6,618,190 discloses an example of forming grooves of various widths in accordance with the values of the amplitudes A on upper surfaces of three-dimensional cells, each having a microscopic size of 0.6×0.25×0.25 μm. Not only is high-precision processing required to form a groove on the upper surface of a physical cell of such microscopic size but extremely high processing precision is required to control the width of the groove accurately to be a width that is in accordance with the value of the amplitude A.